It has long been recognized that the degree of oil recovery by water flooding underground formations decreases as the viscosity of formation oil increases. Under unfavorable viscosity conditions the water, which is highly liquid as compared to oil, tends to finger from an injection well towards a production well so that only a small fraction of the total formation is swept by the oil-displacing water. The swept proportion of the formation and the subsequent oil recovered may be increased considerably when the viscosity of the flooding water is significantly increased by dissolving high molecular weight, water soluble polymers therein.
There are only a few classes of polymers that are suitable for such polymer flooding. In their order of technical importance these are the polyacrylamides, which are mostly partially hydrolyzed; the polysaccharides, which are prepared biologically; and the cellulose ethers, in particular hydroxyethyl cellulose. Furthermore, certain copolymers such as those disclosed in European Patent No. A2-0044508 and prepared from the monomers vinylsulfonate, vinylacylamide and acrylamide may also be used. The use of each of these classes of polymers has advantages and disadvantages which must be weighed for particular applications.
Polyacrylamides are long chain polymers of acrylamide usually having a molecular weight of about 2 to 10 million or more. In partially hydrolyzed polyacrylamides, a portion of the amide groups are saponified into carboxylate groups producing a relatively inexpensive product. The desirable viscosity increase with respect to fresh water by a factor of about 10 to about 40 may be achieved in concentrations of about 0.3 to about 1 kg/m.sup.3 To obtain the same viscosity increases in saline water, concentrations of about 1.5 to about 4 kg/m.sup.3 are required. In high salinity waters it may be advantageous to use polyacrylamides having a lower degree of hydrolysis, i.e., a lower proportion of carboxylate groups.
Polysaccharides are condensed linear or branched saccharides with a molecular weight of from about 500,000 to several million, preferably one million and more, produced by organisms such as Xanthomonas Campestris or Fungus Sclerocium. To achieve an increase in viscosity of about 10 to 40 times that of water, concentrations between about 0.4 and about 2 kg/m.sup.3 are required. Viscosity is largely independent of salinity. Because of the higher price of polysaccharides compared to polyacrylamides, economic considerations generally lead to the use of polyacrylamides under low salinity conditions and polysaccharides under high salinity conditions.
Water soluble cellulose ethers, especially hydroxyalkyl cellulose having an alkyl chain length of about one to about four carbon atoms, are also suitable for polymer flooding. Like polysaccharides, hydroxyethyl cellulose has a high tolerance for salt. But because of a limited molecular weight, concentrations of about 2 to 4 kg/m.sup.3 are required for the desired viscosity increase. For these and other reasons the practical significance of the cellulose derivatives is limited.
The copolymers disclosed in European Pat. No. A2-0044508 and derived from the monomers vinylsulfonate, vinylacylamide and acrylamide exhibit a higher electrolyte and heat stability than the before mentioned classes of polymers. For this reason, this group of polymers is especially suitable for polymer flooding at high salinity conditions and high formation temperatures.
Most oil formations contain high salinity waters. For instance, the total salt content in North German formation brine is usually between about 120 and 250 kg/m.sup.3 In accordance with the prior art, such formations may be initially flooded with fresh water followed by a polymer flood with partially hydrolyzed polyacrylamides in fresh water. However, this flooding procedure cannot be universally used. Some oil formations contain clays which swell upon contact with low salinity flooding media, resulting in blockage of the formation. Frequently, fresh water is not available in the required quantity or quality, or disposal means for the produced brine are insufficient. In such cases polymer flooding will be carried out with a brine-insensitive polymer in formation water, although the economic costs are greater than the costs of polyacrylamide flooding in fresh water.
Polymer retention is a serious factor which limits the operability of all polymer flooding processes. The formation pores containing crude oil and formation water side by side frequently have specific surfaces in the range of about 0.1 to 10 m.sup.2/ g, the actual value depending greatly on the type and quantity of the formation clay. All polymers used for tertiary oil recovery operations bond to a greater or lesser degree to formation rock, especially clays, by chemical adsorption.
Furthermore, high molecular weight polymers may also be mechanically retained by being hooked to the rock surface of the formation, or may be lost in dead end pores. Loss through flocculation may also occur. The total polymer loss during flooding is comprised of both losses through chemical adsorption and retention through mechanical effects. It is well known that mechanical retention loss with polyacrylamides is very high.
If the value of polymer lost by retention during the flooding operation in a given oil field would exceed the value of the additional oil to be recovered through the use of the polymer, the project is not economically feasible. A large number of known oil formations cannot be subjected to polymer flooding because of excessive polymer retention values.
Consequently, attempts have been made to inject so-called sacrificial chemicals into the formation either ahead of or together with oil-displacing polymers to inhibit chemical adsorption by preoccupation of adsorption sites. It is an important characteristic of such a sacrificial chemical that it is normally less expensive than the polymers to be protected.
In flooding with polyacrylamides, a "polymer-conserving agent", is proposed by A. C. Uzoigwe, F. C. Scanlon, R. L. Jewett in "Improvements in Polymer Flooding: The Programmed Slug and the Polymer-Conserving Agent", Journal of Petroleum Technology, January 1974, pp. 33-41. However, in spite of the great efforts made towards finding novel and more effective sacrificial chemicals, polymer flooding is still not widely used due to normally unfavourable economic performance caused by excessive retention.
It is known in liquid and gas chromatography that a silicate- or carbonate-containing carrier material can be effectively deactivated by a polyethylene glycol treatment and a subsequent thermal treatment. See, G. Alexander, "Preparation of Glass Capillary Columns", Chromatographia, Vol. 13, No. 10, October 1980, p. 657. But the reference requires a treatment at 280.degree. C., probably attaining deactivation by unidentified products of thermal decomposition.
U.S. Pat. No. 3,692,113 discloses the use of a polyalkylene glycol of high molecular weight, possibly greater than one million, for further increasing the viscosity of the flooding medium. The method is preferably practiced with alkaline earth metals.
According to U.S. Pat. No. 3,882,939, a polyalkylene glycol of a molecular weight of 10,000 to 10,000,000, preferably from 1 to 6 million, may be injected in the formation together with a biopolymer to increase viscosity. The addition of the polyalkylene glycol is intended to influence the flow properties of the resulting solution at higher shear rates, which means that higher apparent viscosities will exist. This achieves improved mobility control of the flooding medium in the vicinity of the well.
U.S. Pat. No. 3,946,822 proposes flooding with a polyalkylene glycol ahead of flooding with a micellar or polymer solution to reduce contact between the displacing fluid, which is sensitive to di- or trivalent ions, and said ions. A polyethylene glycol having a molecular weight of 10,000 to 100,000,000 is preferred, with 1,000,000 to 6,000,000 being most preferred. Preferred water salinity is between the range of 1 to 20 kg/m.sup.3 total salt content.
Under high-salinity conditions the retention of polymers used for tertiary oil recovery is especially excessive. See, J. Klein and A. Westerkamp, "Comparative Adsorption Studies Using a Polyacrylamide and a Polysaccharide Type Polymer in Oilfield Oriented Model Systems Simulating Enhanced Oil Recovery Conditions", Angew. Makromolekulare Chemie 92 (1980), FIG. 7, p. 24.
It is therefore the object of the invention to provide a process which even under high salinity conditions reduces chemical retention to such an extent that polymer flooding may be carried out.